1. Field of the Invention
The present invention relates to asphalt roofing shingles, and protective granules for such shingles, and processes for makings such granules and shingles.
2. Brief Description of the Prior Art
Mineral surfaced asphalt shingles, such as those described in ASTM D225 (“Standard Specification for Asphalt Shingles (Organic Felt) Surfaced with Mineral Granules”) or D3462 (“Standard Specification for Asphalt Shingles Made From Glass Felt and Surfaced with Mineral Granules”), are generally used on steep-sloped roofs to provide water-shedding function while adding an aesthetically pleasing appearance to the roofs. The asphalt shingles are generally constructed from asphalt-saturated roofing felts and surfaced by pigmented color granules, such as those described in U.S. Pat. No. 4,717,614. Pigment-coated mineral rocks are commonly used as color granules in roofing applications to provide aesthetic as well as protective functions to the asphalt shingles. Roofing granules are generally used in asphalt shingle or in roofing membranes to protect asphalt from harmful ultraviolet radiation.
Roofing granules typically comprise crushed and screened mineral materials, which are subsequently coated with a binder containing one or more coloring pigments, such as suitable metal oxides. The granules are employed to provide a protective layer on asphaltic roofing materials such as shingles, and to add aesthetic values to a roof.
In the past, pigments for roofing granules have usually been selected to provide shingles having an attractive appearance with little thought to the thermal stresses encountered on shingled roofs. However, depending on location and climate, shingled roofs can experience very challenging environmental conditions, which tend to reduce the effective service life of such roofs. One significant environmental stress is the elevated temperature experienced by roofing shingles under sunny, summer conditions, especially roofing shingles coated with dark colored roofing granules.
Conventional asphalt shingles are known to have low solar heat reflectance, and hence will absorb solar heat especially through the near infrared range (700 nm-2500 nm) of the solar spectrum. This phenomenon is increased as the granules covering the surface become dark in color. For example, while white-colored asphalt shingles can have solar reflectance in the range of 25-35%, dark-colored asphalt shingles can have solar reflectance of only 5-15%. Furthermore, except in the white or very light colors, there is typically only a very small amount of pigment in the conventional granule's color coating that reflects solar radiation well. As a result, it is common to measure temperatures as high as 77° C. on the surface of black roofing shingles on a sunny day with 21° C. ambient temperature. Absorption of solar heat may result in elevated temperatures at the shingle's surroundings, which can contribute to the so-called heat-island effects and increase the cooling load to its surroundings or energy consumption needs for air conditioning.
This heat absorption problem has been addressed by applying white pigment-containing latex coatings directly onto the shingle surface on the roof. Although such roofs can be coated with solar reflective paint or coating material, such as a composition containing a significant amount of titanium dioxide pigment, in order to reduce such thermal stresses, this utilitarian approach will often prove to be aesthetically undesirable, especially for residential roofs. Another approach is provided by U.S. Pat. No. 2,732,311, which discloses a method for preparing roofing granules having metal flakes, such as aluminum flakes, adhered to their surfaces, to provide a radiation-reflective surface. Additionally, the use of exterior-grade coatings colored by infrared-reflective pigments for deep-tone colors, and sprayed onto the roof in the field, has been proposed. Employing another approach, U.S. Patent Publication 2003/0068469 A1 discloses an asphalt-based roofing material comprising mat saturated with asphalt coating and a top coating having a top surface layer that has a solar reflectance of at least 70%. The high reflectance of the top surface layer is achieved by embedding metal flakes or a reflective pigment such as titanium dioxide or zinc sulfide in surface layer (paragraph 48). Alternatively, minerals with high solar reflectance can be selected and employed as roofing granules. For example, U.S. Patent Publication 2003/0152747 A1 discloses the use of granules with solar reflectance greater than 55% to enhance the solar reflectivity of asphalt based roofing products. U.S. Patent Publication 2005/0072114 A1 discloses solar-reflective roofing granules having deep-tone colors are formed by coating base mineral particles with a coating composition including an infrared-reflective pigment. Color is provided by colored infrared pigment, light-interference platelet pigment, or a metal oxide. U.S. Patent Publication 2005/0072110 A1 discloses an infrared-reflective material applied directly to the bituminous surface of a roofing product to increase the solar heat reflectance of the product, even when deep-tone roofing granules are used to color the product. The infrared-reflective material can be applied as a powder or in a carrier fluid or film, and can be applied along with infrared-reflective roofing granules.
Thin metal films with thickness ranging from several Angstroms to the order of 100 Angstrom are known to have transparency in the visible light spectrum yet be reflective in the near infrared (“NIR”) or infrared (“IR”) range, such as disclosed, for example, in U.S. Pat. NO. 6,680,134. Such thin metal films have been used in architectural glasses or specialty windows to reflect heat in the NIR or IR ranges.
There is a continuing need for roofing materials, and especially asphalt shingles, that have improved resistance to thermal stresses while providing an attractive appearance. In particular, there is a need for increased solar heat reflectance to reduce the solar absorption of the shingle, while maintaining the aesthetic values of the system.